1. Field of the Invention
The present invention relates generally to aerodynamic control systems and, more particularly, to a system for providing active individual fluid foil control for primary flight control and maneuverability, vibration reduction and noise reduction via a trailing-edge flap.
2. Description of Prior Art
There has been much interest recently in the use of trailing-edge flap (TEF) systems for rotorcraft to provide active individual blade control for vibration reduction. It is possible to provide higher harmonic control of each helicopter rotor blade by directly mitigating the vibrations created by time varying forces and moments. It is also possible to accomplish these results using the same actuation system as is used for primary control of the aircraft. A dual-function actuation system would allow for elimination of the swashplate with its significant drag, complexity, and maintenance penalties. Leishman, J., The Helicopter Thinking Forward, Looking Back, College Park Press, 2007, pp. 101-102.
One of the challenges to realizing these goals is the lack of an appropriately scaled actuation system. The small size and high centrifugal loads experienced in helicopter main rotor blades makes for a highly constrained design space. The challenge is made even more difficult by the need to minimize the weight of any such system. Conventional hydraulic or pneumatic cylinders are heavy, require considerable space, and, in any event, cannot attain the control frequencies necessary for vibration reduction. Consequently, there has been considerable interest in the use of smart materials as the driving elements of these systems.
Piezoelectrics, electrostrictives, magnetostrictives, and shape memory alloys have all been investigated as possible replacements for conventional fluid driven control systems for fixed wing and rotary aircraft. Piezoelectric systems have attracted the most attention and have been the most successful to date. Niezrecki, C., Brei, D., et al., “Piezoelectric Actuation: State of the Art,” The Shock and Vibration Digest, 2001, Vol. 33, 269. A full-scale whirl test and the first ever flight test of a smart rotor equipped helicopter have recently taken place with piezo-based active flap systems. See, Straub et al., “Development and whirl tower test of the SMART active flap rotor,” SPIE intl. Symposium in Smart Structures and Materials, San Diego, Calif., Mar. 14-18, 2004; Dieterich et al., “Trailing Edge Flaps for Active Rotor Control Aeroelastic Characteristics of the ADASYS Rotor System” American Helicopter Society 62th Annual Forum, Phoenix, Ariz., USA, 9-11 May 2006. Unfortunately, piezoelectrics are known to suffer from some inherent limitations including, among others, their small actuation strains (<0.2%), brittleness, and high cost. More importantly for present purposes, piezoelectrics have insufficient actuation authority for primary control of full-scale rotor systems.
Another possibility that has shown promise is Fluidic Artificial Muscles (FAMs). Fluidic artificial muscles (also known as artificial muscle actuators, or McKibben artificial muscles, among other names), are simple mechanical actuators that harness pressurized fluid (air, water, oil, etc.) to generate significant forces and deflections. Fluidic artificial muscles commonly comprise an inner elastomeric fluid bladder that is sealed on each end to allow for pressurization and surrounded by a stiff braided sleeve, though co-cured bladder-braid, layered helical windings, and straight fibers are also options for sleeve design.
In operation, pressurization of the inner elastic bladder will produce either contractile or extensile force and motion due to the interaction with the braided sleeve. The bladder is pressurized with an operating fluid such as air or oil causing an inflation and expansion of the bladder and the braided sleeve which surrounds it. The fixed length of the stiff sleeve fibers generates a contractile or extensile force along the main axis of the actuator in addition to relative motion between the two end fittings. The direction of force and motion is dependent on the initial angle between the filaments of the braided sleeve. For a contractile actuator, the bladder expansion is radial, whereas for an extensile actuator the bladder expansion is primarily axial. The generated force and motion is transferred to an external system via the end fittings. Fluidic artificial muscle actuators of this type have been known in prior patent publications. A related device was disclosed in April 1957 in U.S. Pat. No. 2,789,580. Many different designs have been disclosed over the years (U.S. Pat. Nos. 2,844,126, 4,615,260, 4,733,603, 4,751,869, 4,733,603 and 6,349,746), but only in the context of robotics or industrial automation, not aerospace engineering. The latter has only recently become a viable application due to advances in FAM design that enable lighter weight and higher fatigue life. See, for example, Applicants' co-pending application publication no. 2009/0301292 which is incorporated herein by reference in its entirety and recently issued U.S. Pat. No. 7,837,144 also incorporated herein by reference.